In continuous production of desired moldings by injecting plasticized synthetic resin from the cylinder of an injection molder through the nozzle to fill in the cavity of a mold connected to the nozzle, it is essential to inject the resin by accurate amount in term of weight corresponding to the capacity of the cavity or the volume of the intended product. If the filling weight is greater than the required value, excess resin is forced out through the parting line of the mold to form flash. Inversely, if the filling weight is smaller than the required value, recesses are formed on the molded surface, resulting in a defective product with sinks.
Conventionally, for continuous production of moldings of a constant quality, it is known that the weight of plasticized synthetic resin injected from an injection molder to fill in a mold cavity is detected on the basis of a position of a screw in the injection molder. The followings are the methods:
i) The filling weight is detected on the basis of the screw position in the injection molder before resin injection into the mold cavity. PA0 ii) The filling weight is detected on the basis of either of the following screw positions upon completion of dwelling following the resin injection into the mold cavity: PA0 iii) The filling weight is detected on the basis of the difference between the screw position before resin injection into the mold cavity and that after dwelling following the injection. PA0 A. The check valve at the front end of the screw is in the closed state when obtaining the positional value of the screw or the resin pressure value in the cylinder corresponding to the positional value. PA0 B. The pressure of the plasticized synthetic resin is maintained at a specified value when obtaining the positional value of the screw or the resin pressure value in the cylinder corresponding to the positional value. PA0 C. The pressure of the plasticized synthetic resin must not influence molded products when obtaining the positional value of the screw or the resin pressure value in the cylinder corresponding to the positional value. PA0 (a) preliminarily obtaining the formula for relation between the pressure value and specific volume value of plasticized synthetic resin as a characteristic equation for the resin; PA0 (b) providing, between the cylinder and the mold cavity, a flow path closing mechanism capable of blocking the flow of plasticized synthetic resin between the cylinder and the cavity; PA0 (c) closing the flow path closing mechanism to block the resin flow before injecting the resin from the cylinder into the cavity; PA0 (d) applying a specified pressure to the screw with the resin flow blocked so as to effect a balancing movement of the screw; PA0 (e) obtaining the positional value of the screw at a first stop position at which the screw stops its balancing movement, and the resin pressure value determined by the pressure applied to the screw at the first stop position; PA0 (f) opening the flow path closing mechanism to allow plasticized synthetic resin to flow through; PA0 (g) applying pressure to the screw with the flow path closing mechanism open, to inject the resin into the mold cavity; PA0 (h) closing the flow path closing mechanism to block the resin flow upon completion of injection and dwelling; PA0 (i) applying a specified pressure to the screw with the resin flow blocked so as to effect another balancing movement of the screw; PA0 (j) obtaining the positional value of the screw at a second stop position at which the screw stops its balancing movement, and the resin pressure value determined by the pressure applied to the screw at the second stop position; and PA0 (k) obtaining the weight of the plasticized synthetic resin injected to fill in the mold cavity, on the basis of the formula for relation between resin pressure value and specific volume value, as well as the positional values of the screw at the first and second stop positions and the resin pressure values determined by the pressures applied to the screw at the first and second stop positions.
(1) The position of the screw at the forward limit for dwelling. PA1 (2) The position of the screw relieved from the dwelling pressure after dwelling. PA1 G': filling weight PA1 V': inside capacity of cavity PA1 A: inside cross sectional area of cylinder PA1 S.sub.I : positional value of screw before injection PA1 S.sub.H : positional value of screw after injection and dwelling PA1 P.sub.I1 : resin pressure value for S.sub.I PA1 P.sub.H1 : resin pressure value for S.sub.H PA1 V(P.sub.I1): specific volume value for S.sub.I and P.sub.I1 PA1 V(P.sub.H1): specific volume value for S.sub.H and P.sub.H1
In any of these conventional methods, however, the filling weight can be detected with a limited accuracy and so forth because of the following reasons:
Method of i)
The pressure of plasticized synthetic resin in the injection molder cylinder is not controlled nor monitored until immediately before resin injection through the nozzle of the molder. Therefore, the pressure of the plasticized synthetic resin does not become constant in the case of the plasticized synthetic resin having compressibility. Particularly where the screw of the injection molder is retreated forcedly for reducing the resin pressure in the cylinder so as to prevent the resin from leaking through the injection molder nozzle, the resin pressure fluctuates due to air drawn through the nozzle end.
The check valve provided at the front end of the screw is open before injection because no pressure is applied to the screw. When the check valve is closed under pressure applied to the screw for injection, small amount of resin flows back in the cylinder. This also results in the poor detection accuracy of this method.
Method of ii)
For the case (1)
The dwelling pressure is varied to meet various molding conditions for different products. The compression amount of compressible plasticized synthetic resin varies depending on the dwelling pressure.
A high detection accuracy could be achieved by comparing the resin amount in the injection molder cylinder under a constant dwelling pressure. However, since the dwelling pressure is one of the essential molding conditions that determine the shape and dimensions of moldings, it is impracticable to maintain a constant dwelling pressure.
For the case (2)
After application of pressure to the screw for dwelling is stopped, it takes certain time for the plasticized synthetic resin in the injection molder cylinder to be relieved from the pressure. Since the time required for this purpose varies depending on the kind and temperature of the plasticized synthetic resin, it is difficult to know whether or not the resin has been relieved completely from pressure.
Although it takes certain time to remove a dwelling pressure from the plasticized synthetic resin, it is general to assign rather short time for this purpose to avoid reduction in the productivity in the actual molding operation.
Method of iii)
The difference in the screw position indicates the volume of injected resin. However, the resin volume changes with the resin temperature and/or the pressure applied to the screw, and is not proportional to the weight of the injected resin. Moreover, the resin temperature and the pressure applied to the screw are among variable molding conditions. So, the difference in the screw position is not a good measure for the weight of injected resin. Furthermore, in continuous molding, since the hydraulic oil temperature and ambient temperature change with time, it is difficult to maintain a constant resin temperature and/or constant pressure to be applied to the screw. Even if the resin volume fluctuation caused by change in these conditions is detected in term of the change in screw position, it is impossible to detect defective products, forecasting such change in the molding conditions; normal products would be rejected as defective products, thus decreasing the productivity.
To solve these problems, the object of the present invention is to provide a method of detecting the filling weight for an injection molder, which permits highly accurate detection, on the injection molder side, of the weight of plasticized synthetic resin injected from the injection molder to fill in a mold cavity.